What does this message mean?
You try to open an email archive in Apple Mail and you get the message 'no valid mbox files were found'. Apple Mail then refuses to import, even though the file is clearly there. That is confusing, because you know your emails are somewhere inside that archive.
The message does not mean your emails are gone. It only means Apple Mail does not recognise the layout of the file or folder as something it can import. In almost every case the content is still fully present, you just need to reach it a different way.
Why does this happen?
There are a few common causes. It helps to understand how Apple Mail stores an archive on your disk.
Apple Mail does not save a mailbox as one loose .mbox text file. It saves it as a .mbox package. In Finder that looks like a single icon, but it is really a folder. Inside that folder sits a text file named 'mbox' or 'Messages' that holds the actual emails, plus extra files for the index. If you point Apple Mail at that whole package during import, it sometimes expects a flat mbox instead and gives up.
The most common causes at a glance:
- You point at a .mbox package, but at that moment Apple Mail expects a loose text file (or the other way around).
- The folder structure is not what Apple Mail wants. It prefers a folder that contains the .mbox packages side by side, not a single inner file on its own.
- The package was copied incompletely. When dragging between disks or through a cloud service, the inner 'Messages' text file or the index can be left behind.
- The file comes from another program (Thunderbird, Outlook, Google Takeout) and has a slightly different layout than Apple Mail expects.
- The archive is a .mbox.gz (compressed, as from Google Takeout) and has not been unpacked yet.
Package or file?
An Apple Mail .mbox package is really a folder. Right-click it in Finder and choose 'Show Package Contents' to see the separate parts, including the text file that holds your emails.
Quick fix: read your archive with Mbox Viewer
If you mainly just want to reach your emails without wrestling with the Apple Mail import, the simplest route is to open the archive in Mbox Viewer by Cloud Captains. It is a Chrome extension that reads mbox and email files straight in your browser. You do not have to import anything and nothing goes to the cloud.
Mbox Viewer opens .mbox and .mbx (Thunderbird, macOS Mail), .eml and .emlx (Apple Mail), .msg (Outlook), Maildir folders and .mbox.gz from Google Takeout (which it unpacks automatically). You simply drag the file or folder into the window.
Opening your Apple Mail archive in Mbox Viewer
- Install Mbox Viewer by Cloud Captains from the homepage https://mbox-viewer.online and open the extension.
- Find your archive in Finder. For a .mbox package, right-click it and choose 'Show Package Contents'.
- Drag the whole .mbox package into the Mbox Viewer window, or drag the inner text file named 'mbox' or 'Messages' if the package itself is not recognised.
- Wait for the message list to appear on the left. Click a message to read it on the right.
- If it still does not work, drag the parent folder that holds all your mailboxes into the window in one go.
Everything happens on your own device. Your emails are read locally in your browser and stored in IndexedDB on your computer. Nothing is sent to a server and there is no telemetry. If you want the data gone later, you clear it through Settings, Clear database.
External images stay blocked
While reading, Mbox Viewer blocks external images by default, so tracking pixels cannot reveal that you opened an old email. If you do want to see the images in a specific message, you load them per message with a button.
Getting it into Apple Mail after all
If you really want to import the archive into Apple Mail itself, try the following approach. The trick is to hand Apple Mail exactly the structure it expects.
Making the Apple Mail import succeed
- Create an empty folder on your desktop, for example 'Import'.
- Put the complete .mbox packages you want to import inside it, so not the inner files on their own, but the full packages.
- Open Apple Mail and choose File, Import Mailboxes from the menu bar.
- For the import type, choose 'Apple Mail' if the archive comes from Apple Mail, or 'Files in mbox format' if it comes from another program.
- Point at the 'Import' folder (not a single loose file) and let Apple Mail find the mailboxes.
- If the message returns, check that each .mbox package still contains its inner 'Messages' text file using 'Show Package Contents'.
Copy the whole package, not half of it
If the message keeps coming back, there is a good chance part of the .mbox package was left behind during copying or syncing. Copy the package again as one whole, preferably locally and not through a cloud service that treats folders differently.
What if the archive is compressed?
If your archive comes from Google Takeout, it is often a .mbox.gz, which is a compressed mbox. Apple Mail cannot handle that directly and so reports no valid mbox file. Mbox Viewer unpacks .mbox.gz automatically the moment you drag it into the window, so you do not need to unpack anything yourself.
If the archive comes from Outlook, you probably have .msg files, not mbox. Mbox Viewer opens those directly too.
Checking that your archive is intact
If you want to be sure your archive is complete and was not damaged along the way, the forensic tab in Mbox Viewer helps. Per message it shows the real headers, the authentication result (SPF, DKIM, DMARC), the transport chain and hashes per attachment. That way you confirm not only that an email is readable, but also that it is unchanged. This is useful when you keep the archive for legal or business reasons.
Are my emails lost when I get this message?
No. The message is about the layout Apple Mail expects during import, not about the content. In almost every case your emails are still fully present in the file or package. You just need to open them a different way, for example with Mbox Viewer by Cloud Captains.
What is the difference between a .mbox file and a .mbox package?
A loose .mbox file is one text file with all the emails one after another. An Apple Mail .mbox package looks like a single icon, but is really a folder containing a text file (often named 'mbox' or 'Messages') plus index files. During import Apple Mail sometimes expects the one and sometimes the other, which can trigger the message.
Do I have to upload my archive to view it in Mbox Viewer?
No. Mbox Viewer works fully locally and offline. Your files are read inside your own browser and stored in IndexedDB on your device. Nothing goes to a server, there is no telemetry, and the Chrome permissions are empty. You clear the data whenever you like through Settings, Clear database.
My archive is a .mbox.gz from Google Takeout, what now?
That is a compressed mbox. Apple Mail cannot import it directly, but Mbox Viewer unpacks .mbox.gz automatically the moment you drag it into the window. So you do not need to unpack anything yourself.
Does this also work with emails from Outlook or Thunderbird?
Yes. Besides Apple Mail files, Mbox Viewer also opens .mbox and .mbx from Thunderbird, .msg from Outlook, .eml and .emlx, and Maildir folders. You drag them all into the window the same way.
Want to know more about what you can do with your archive, visit https://mbox-viewer.online.